(1) Field of the Invention
The present invention relates to a metal hydride storage cell having excellent charge and discharge cycle characteristic and a method of producing such a storage cell.
(2) Related Art
Along with the rapid development of the recent technologies, electronic devices are getting portable and cordless. As power for such devices, there is an increasing demand for compact secondary cells having high energy density and efficiency. Under such circumstances, metal hydride storage cells in which hydrogen absorbing alloy is used for the negative electrode are getting more and more attentions, as they are clean energy source having higher capacity and density compared to nickel-cadmium storage cells and lead storage cells.
The metal hydride storage cells require long charge and discharge cycle life as well as the high capacity. Japanese Laid-open Patent Application No. 6-283171 discloses determining the cumulative particle size distribution of the hydrogen absorbing alloy used in the negative electrode so as to obtain a metal hydride storage cell which has small increase of internal pressure during charging, long charge/discharge cycle life, and excellent rapid discharge characteristic. Specifically, it is to use alloy powder having the following cumulative particle size distribution for the negative electrode of the metal hydride storage cell: supposing that the diameter of the particle of the hydrogen absorbing alloy powder is expressed as R, which is the diameter of the two dimensions, the breakdown of the cumulation of the powder is as follows: 0% regarding R less than 10 xcexcm; 5% regarding R less than 20 xcexcm; 5-12% regarding R less than 30 xcexcm; 20-30% regarding R less than 50 xcexcm; 60-80% regarding R less than 100 xcexcm; and 95-100% regarding R less than 300 xcexcm.
As is disclosed in the above reference, the charging and discharging cycle life can be improved to some degree by making the particle size distribution of the hydrogen absorbing alloy rather sharp when used for the electrode which is to be used for a cell.
However, in reality, charge and discharge characteristic are affected by the charging and discharging conditions. The above mentioned technique does not necessarily guarantee the improvement of the cycle characteristic.
It is an object of the present invention to certainly provide a metal hydride storage cell which has excellent cycle characteristic and a method of producing such a metal hydride storage cell. The object can be achieved by the following features.
(1) A metal hydride storage cell using a hydrogen absorbing alloy for an electrode, the metal hydride storage call characterized by particle size distribution of the hydrogen absorbing alloy after charging and discharging under a given condition being y/x less than 0.90, with x being an average particle diameter and y being a standard deviation .
(2) A method of producing a metal hydride storage cell, comprising the steps of: assembling the cell by inserting an electrode group into a given casing into which electrolyte is poured, the electrode group comprising a positive electrode, a negative electrode which has the hydrogen absorbing alloy, and a separator sandwiched in-between; and giving at least one-cycle charging and discharging to the cell under a condition in which particle size distribution of the hydrogen absorbing alloy after charging and discharging becomes y/x less than 0.90, with x being an average particle diameter and y being a standard deviation.
The particle size distribution is measured by laser diffraction method.
There are some methods of adjusting the particle size distribution after the charging and discharging, such as adjusting particle size distribution of the hydride absorbing alloy used in the electrode, adjusting charging/discharging conditions, and heat treating the hydrogen absorbing alloy; a combination of the methods is also possible.
Cycle life can be improved for sure by determining the particle site distribution after the charging and discharging rather than by determining the particle size distribution before the charging and discharging. The reason can be explained as follows.
Generally speaking, the size of the hydrogen absorbing alloy particles should be uniform in size during the charging and discharging so that electrochemical reaction is not damaged. More specifically, where the particles are uniform in size, i.e., uniform in the surface area, then the same amount of the surface area is activated for each particle. Thus, each particle undergoes the electrochemical reaction in an uniform way.
On the other hand, when the particles are not uniform in size, since large particles are inert they do not contribute to charging and discharging reaction. And as small particles selectively repeat charging and discharging, which makes small particles even smaller, cycle life is shortened due to the fall out of the alloy from the electrode or oxidation/ deterioration of the alloy.
Therefore, as the initial particle distribution value, i.e., initial y/x value, is smaller, in other words, as the particle distribution curve is sharper, the cycle characteristics are better.
As the charging and discharging cycle proceeds, the particle size distribution y/x value becomes large, in other words, particle size distribution tend to become broad. However, the particle size rapidly becomes smaller at the initial stage. Especially, during activation, as depth of charge and discharge is deep, the particle size becomes rapidly small. After that, the particles become gradually small. Therefore, it can be assumed that there is a strong correlation between the particle size distribution and the cycle life after the initial charging and discharging compared to the correlation between the initial particle size distribution and the cycle life.
Moreover the inventor has found from experiments that the cycle life significantly improves when the particle size distribution y/x value (x=average particle size, y=standard deviation) after at least one-cycle charging and discharging is below 0.90.
As mentioned above, according to the present invention, by making the particle size distribution y/x value below 0.90 after cycles of charging and discharging under given conditions, a metal hydride storage cell which has excellent cycle life compared to conventional ones can be obtained.